Updated project metadata. MCM8 has emerged as a core gene in reproductive aging and is crucial for meiotic homologous recombination repair. It also safeguards genome stability by coordinating the replication stress response during mitosis, but its function in mitotic germ cells remains elusive. Here we found that the number of primordial germ cells (PGCs) decreased dramatically after MCM8 deficiency due to proliferation defects. We further demonstrated that MCM8 interacted with two known helicases DDX5 and DHX9 in PGCs, and loss of MCM8 led to R-loop accumulation by reducing the retention of these helicases at R-loops, thus inducing genome instability. Cells expressing premature ovarian insufficiency-causative mutants of MCM8 with decreased interaction with DDX5 displayed increased R-loop levels. These results suggest that MCM8 interacts with R-loop-resolving factors to prevent DNA damage in PGCs, thus maintaining PGC proliferation and establishing the reproductive reserve. Our findings thus reveal a novel role for MCM8 in PGC development and improve our understanding of reproductive aging caused by genome instability in mitotic germ cells.